A new technique developed by a University of Colorado Boulder team converts sunshine and water directly into usable fuel. The technique involves concentrating sunlight in a solar tower to achieve temperatures high enough to drive chemical reactions that split water into its constituent oxygen and hydrogen molecules. In this way, the team says it should be able to cheaply produce massive amounts of hydrogen fuel.
The team's solar thermal system concentrates sunlight off a vast array of mirrors into a single point at the top of a tall tower to produce very high temperatures. When this heat is delivered into a reactor full of metal oxides, the oxides heat up and release oxygen. The reduced metal oxide now gains a chemical composition that makes it ready to bind with oxygen atoms. Introducing steam into the reactor, which can also be produced by heating water with sunlight, causes the compound to draw oxygen atoms out of the water molecules, leaving behind hydrogen molecules that can be collected as hydrogen gas.
While the concept of using an array of mirrors to concentrate sunlight into a single point at the top of a tall tower is nothing new, being the same technique used in solar thermal tower power plants, there are certain key differences here. Typically, sunlight is concentrated about 500 to 800 times in standard solar power tower designs to reach temperatures of about 500º C (932 º F) and produce steam that drives a turbine to generate electricity. However, splitting water requires temperatures of around 1,350º C (2,500º F), which is hot enough to melt steel.
"You need this high temperature both to give you the driving force to drive the chemical reactions and also the kinetics to make the reactions go fast enough to make the process practical," says Charles Musgrave, Professor of Chemical and Biological engineering at CU-Boulder.
To get those kinds of temperatures, the team added additional mirrors within the tower to further concentrate the sunlight onto the reactor and the active material. While it isn't too different in principle from using a magnifying glass to focus sunlight onto a piece of paper to get it to burn, this setup allows the reflected sunlight to be concentrated by up to 2,000 times. "We are trying to use sunlight to drive chemical reactions that require higher temperatures than combustion," says Musgrave.
The big breakthrough came about when the team discovered certain active materials that allowed both these chemical reactions (reducing the metal oxide and re-oxidizing it with steam) to occur at the same temperature.
Though there aren't any working models, conventional theory dictates that a change in temperature is necessary to make the two different reactions occur – a high temperature for reducing the oxide and a low temperature for re-oxidation. Instead, the introduction or the absence of steam is used to drive the different reactions and certain unique properties of the metal oxide compounds used makes this possible.
"We determined that both reactions could be driven at the same temperature of about 2,500° F (1,371° C)," Musgrave told us. "Even though we run at a constant and lower temperature we still generate more hydrogen than competing processes."
Alan Weimer, the research group leader at CU-Boulder says that eliminating the time and energy required for temperature swings lets them make more hydrogen in a given amount of time. To produce even more hydrogen fuel they'd only need to increase the amount of material in the reactor. "In many respects, our approach is out of the box where prior work was inside of the box using the temperature swing," he adds.
According to the team, huge solar plants spread across many acres could produce much more fuel per acre than biofuels for the same amount of acreage. Another advantage that this process has over other renewable technologies, such as wind and photovoltaics, is that it directs sunlight to directly drive chemical reactions to produce fuel for use in combustion engines or fuel cells. In contrast, photovoltaic processes first convert sunlight into electricity, reducing overall efficiency.
"Our objective is to produce hydrogen (H2) at $2/kg H2," Weimer tells Gizmag. "This is equivalent to about US$2/gallon (3.7 L) of gasoline based on mileage in a fuel cell car versus a combustion engine today." The team believes that a site with five 223 m (732 ft) tall towers and about two million sq m (21.5 million sq ft) of heliostats on 485 ha (1,200 acres) of land could generate 100,000 kg (222,460 lb) of hydrogen per day, which is enough to run over 5,000 hydrogen-fuel cell buses daily.
Though the technology has the potential to be a game-changer in pushing the hydrogen economy forward, commercialization might still be several years away thanks to continuing stiff competition from fossil fuels.
The National Science Foundation and the U.S. Department of Energy supported the research and a paper on the system was published in the Aug. 2 issue of Science.
Source : University of Colorado Boulder
Update: This story was amended on Aug. 7, 2013 to correct an error in figures given to us that related to land area required to generate 100,000 kg of hydrogen per day. We were originally told it was 120,000 acres, when the correct figure is 1,200 acres. Our apologies.